Generally speaking, when signal transmits through a transmission channel, signal may attenuate and result in amplitude decay or waveform distortion. Therefore, an equalizer is usually used at the receiving end for compensating the attenuation caused by the transmission channel. Besides, a signal may comprise a plurality of signal components respectively corresponding to different frequency bands. Signal attenuation of each of the signal components may be different and thereby different compensations executed by the equalizer for different signal attenuation are necessary. The conventional equalizer usually multiplying the signal components respectively corresponding to the high and low frequency bands with different gains and then adding these signal components up to compensate the attenuation. However, because signal attenuation varies with different transmission distances, it's not suitable for the equalizer using the same setting to compensate signal attenuation caused by different transmission distances such as the lengths of a short cable and a long cable. Therefore, the problem of compatibility exists.
Another conventional equalizer is an adaptive equalizer, whose operation includes two modes: training and tracking. For an adaptive equalizer, a test sequence of a predetermined length is sent from a transmitting end to a receiving end with the adaptive equalizer. The adaptive equalizer can adjust corresponding parameter settings according to the received signal and the known sequence, and determine which one is the optimum setting through correctness of the bit-error-rate (BER) or the cyclic redundancy check (CRC). However, if the transmitting end of the channel can't send a test sequence, the optimum parameter setting applicable to the adaptive equalizer can't be found, not mentioning the compensation for the attenuation to the signal caused by the transmission channel.